[PATCH] Indirect call target profiling related profile reader/writer changes

[Philip Reames]

On 04/14/2015 11:56 AM, betulb at codeaurora.org wrote:
>> On 04/10/2015 09:25 AM, betulb at codeaurora.org wrote:
>>>> On 04/09/2015 11:06 AM, Betul Buyukkurt wrote:
>>>>> In http://reviews.llvm.org/D8908#153838, @reames wrote:
>>>>>
>>>>>> Have the IR level construct patches made it up for review?  If so,
>>>>>> can
>>>>> So far I've posted two patches. These two patches should apply cleanly
>>>>> to the tip, working with the present profile infrastructure. The next
>>>>> set of patches will be the enabler ones: i.e. three more patches one
>>>>> for
>>>>> each of clang, llvm and compiler-rt. Clang patch will be up for review
>>>>> later today.
>>>>>
>>>>>> you send me a link?  I managed to miss them.
>>>>> So far there is this patch and the instrinsic instruction definitions:
>>>>> http://reviews.llvm.org/D8877. All patches are necessary for getting
>>>>> the
>>>>> IC targets and having them displayed by the llvm-profdata.
>>>> Ok, I'm really not convinced that the instrumentation code needs to be
>>>> or should be an intrinsic.  This seems like something which should be
>>>> emitted by the frontend and optimized like any other code.  To say this
>>>> a different way, my instrumentation is going to be entirely different
>>>> than your instrumentation.
>>>>
>>>> Having said that, I really don't care about this part of the proposed
>>>> changes since they aren't going to impact me at all.  I'm am
>>>> specifically not objecting to the changes, just commenting.  :)
>>>>>> I'm assuming this will be some type of per call site metadata?
>>>>> We do assign metadata at the indirect call sites. Format looks like as
>>>>> follows:
>>>>>
>>>>> !33 = metadata !{metadata !"indirect_call_targets", i64
>>>>> <total_exec_count>, metadata !"target_fn1”, i64 <target_fn1_count>,
>>>>> metadata !"target_fn2”, i64 <target_fn2_count>, ….}
>>>>>
>>>>> Currently, we're recording only the top most called five function
>>>>> names
>>>>> at each indirect call site. Following the string literal
>>>>> “indirect_call_targets” are the fields  <total_exec_count> i.e. a
>>>>> 64
>>>>> bit value for the total number of times the indirect call is executed
>>>>> followed by the function names and execution counts of each target.
>>>> This was the part I was trying to ask about.  I really want to see
>>>> where
>>>> you're going with this optimization wise.  My naive guess is that this
>>>> is going to be slightly off for what you actually want.
>>>>
>>>> Assuming you're going for profile guided devirtualization (and thus
>>>> inlining), being able to check the type of the receiver (as opposed to
>>>> the result of the virtual lookup) might be advantageous.  (Or, to say
>>>> it
>>>> differently, that's what I'm used to seeing.  Your approach might be
>>>> completely reasonable, it's just not what I'm used to seeing.)  Have
>>>> you
>>>> thought about the tradeoffs here?
>>> Not sure if I understood the problem here,
>> First, I am not trying to say there is a problem with your approach; I
>> am only saying that it's not what I would have expected based on past
>> experience.  You may be entirely correct in your approach, you just need
>> to convince me of that.  :)
>>> however, we're recording both
>>> the target address and the addresses/names of the instrumented functions
>>> during the execution of the instrumented binary. During profile reading
>>> these addresses are used to match the target addresses to corresponding
>>> functions.
>> Ok, let's start from the basics.  For profile guided devirtualization,
>> you're constructing a cache from (something) to function pointer and
>> using that cache lookup to enable inlining of the hot target.  You have
>> two standard choices on what to use as your cache key: the result of the
>> virtual lookup and the inputs to the virtual lookup.
>>
>> Option 1 - Inputs to virtual lookup
>> if ((receiver, vtable index) == what I predicted)
>>     tartget_I_predicted(); // inline me!!
>> else {
>>     target = full virtual dispatch();
>>     target();
>> }
>>
>> Option 2 - result of virtual lookup
>> target = full virtual dispatch();
>> if ('target' == what I predicted)
>>     tartget_I_predicted(); // inline me!!
>> else {
>>     target();
>> }
>>
>> You seem to be proposing option 2.  I'm saying that I'm used to seeing
>> option 1 used.  Both approaches have their appeal, I'm just asking you
>> to explain *why* you've chosen the one you apparently have.
> Not all indirect calls occur from C++ like codes. We're profiling and
> optimizing out indirect calls from C codes as well. We're seeing up to 8%
> gains on individual benchmarks in spec. This was measured on our platform.
This answers my original question and is a good point.  I was definitely 
looking at this through a virtual dispatch centric view.

I have no objection to supporting profiling/predication of arbitrary 
indirect call sites.  As someone else pointed out in the discussion, 
there's no reason we can't specialize for virtual call profiling and 
support the generic profiling as a fallback.  That seems like a good way 
forward.  The details still need to worked out, but the general approach 
makes sense.

p.s. My bandwidth to devote to this topic has pretty much disappeared.  
I probably won't be responding further unless specifically requested to.
>
>>> During optimization, target function name should be adequate to
>>> retrieve the function type information which then can be used to compare
>>> the types of the arguments as well as the return type w/ the call site
>>> to
>>> ensure the match.
>> Er, I'm now really confused.  With a properly constructed check, there
>> should be no runtime checking of types required.  Can you explain why
>> you think this is needed?
>>> To clarify, our interest at this time is to upstream all the
>>> infrastructure related changes. The optimizations using the profile data
>>> will not be part of our initial set of patches. On the other hand,
>>> agreeing on the metadata format at the call sites would be advantageous
>>> in
>>> terms of supporting multiple profile methods.
>> See my other response to this thread.
>>> Thanks,
>>> -Betul
>>>
>>>
>